This invention relates to a current lead structure for a superconducting electrical apparatus disposed in a cryostat, and particularly to such a structure for connecting the apparatus to an external circuit in the atmosphere at normal temperature.
FIG. 1 shows a conventional current lead structure L associated with a cryostat 1 containing liquid helium 2 and a superconducting electrical apparatus (not shown), similar to that described in Japanese kokai No. 60-173883 published Sept. 7, 1985. The cryostat is disposed in a chamber 3 having a vacuum space 11 to adiabatically isolate it from the external atmosphere. A metal pipe 6 having thermal and electrical insulating material 5 applied to its inner surface has an open lower end inserted in and mounted to a wall 1a of the cryostat, and an upper end closed by a cap 7 projecting outwardly from and fixed to the vacuum chamber 3. A connecting terminal 7a extends through the cap. An elongate conductor 8 is disposed in the insulated pipe 6 and is connected between the terminal 7a and a conductor 9 coupled to the superconducting electrical apparatus. An outlet 10 is attached to the upper end of the pipe 5 to vent or exhaust the vaporized cooling medium, i.e. helium gas. Such gas may be simply vented to the atmosphere, but more typically it would forcibly be withdrawn, i.e. pumped out, reliquefied, and returned to the cryostat in a closed cycle recirculation system.
In such a conventional current lead structure, the superconducting electrical apparatus, such as an induction coil, is cooled by its immersion in the liquid helium 2 within the cryostat to a temperature below 4.2.degree. K., and the vacuum space 11 between the cryostat and the chamber 3 reduces the heat loss to the atmosphere. Moreover, the exhaust of the vaporized but still very cold helium gas from the space 4 within the cryostat, up through the pipe 5 and out through the outlet 10, serves to carry away much of the Joule or I.sup.2 R heat generated in the conductor 8 by the high amplitude current flowing therethrough.
Conflicting considerations arise with such a construction, however. On the one hand, it is desirable that the conductor 8 be as long as possible to maximize its cooling by the exhausting helium gas and attendantly minimize the heat loss to the atmosphere. On the other hand, it is desirable that the conductor 8 be as short as possible to reduce the overall size of the apparatus.